The present invention generally relates to polyolefin products such as polyethylene and polypropylene packaging materials for food and beverages. More particularly, this invention relates to reduction of aldehydes in polyolefins by incorporation of additives capable of reacting with these aldehydes.
Polyolefins, particularly polyethylene and polypropylene, are utilized extensively across a wide range of industries in a variety of product forms, including fibers, films, and three-dimensional structures. A particularly important application for polyolefins is containers, especially for food and beverages. Polyolefin materials often are selected for use in food and beverage packages and closures because they are inexpensive, lightweight, easily modified, and impact resistant. Yet despite these advantages, polyolefins can have certain undesirable attributes. For example, they frequently contribute xe2x80x9coff-tastexe2x80x9d to packaged products, particularly sensitive products such as water. This off-taste commonly is referred to as a xe2x80x9cplasticxe2x80x9d off-taste. It would be highly desirable to reduce or eliminate this off-taste from products packaged in polyolefin materials.
One approach that has been taken to mitigate the off-taste problem in the non-polyolefin polymer polyethylene terephthalate (PET) is to incorporate additives that are scavengers for aldehydes that are produced by degradation of the PET itself during heat processing of the polymer. U.S. Pat. No. 4,837,115 to Igarashi discloses incorporating additives of amine-group terminated polyamides and amine-group containing small molecules into PET, wherein the additives selectively react with acetaldehyde (AA) that is generated in the PET. Igarashi teaches that the amine groups are effective because they can react with AA to form imines, where the amine nitrogen forms a double bond with the AA moiety. Igarashi teaches that essentially any amine is effective. Mills (U.S. Pat. Nos. 5,258,233; 5,650,469; and 5,340,884) and Long (U.S. Pat. No. 5,266,416) disclose the use of various polyamides, especially low molecular weight polyamides, while Turner and Nicely (PCT WO 97/28218) disclose the use of polyesteramides. These polyamides and polyesteramides are believed to react with AA in the same manner as described by Igarashi. While these AA scavengers are effective at reducing the AA content of melt-processed PET, they suffer from their own drawbacks. In particular, relatively high loadings of the polyamides are needed to effect significant AA reductions, and a significant yellowing of the PET occurs from incorporation of these amine-containing additives. This coloring is believed to be due to the inherent color of the imine group, which for most applications is undesirable because most polyolefin articles in use today are clear and uncolored.
U.S. Pat. No. 5,663,223 to Teumac discloses using a polymeric liner that includes inorganic sulfites or organic hydrazides as flavor protectant compounds. These compounds are added to scavenge oxygen that otherwise could react with the polymer or additives therein to form off-taste producing aldehydes. Incorporation of these compounds into polyolefins is difficult, however, because these protectant compounds have an extremely low thermal stability. Furthermore, these compounds may themselves contribute to an off-taste in sensitive products, and hydrazides react with aldehydes to form hydrazones, which often are highly colored. Such colors oftentimes are unwanted, as many applications require or desire that the polyolefin packaging be white or clear.
A simple and economical method is therefore needed for reducing aldehyde content in polyolefin products without using aldehyde-scavenging agents that impart an off-taste to products packaged in containers made from polyolefins or that impart a color to such polyolefin containers.
It is therefore an object of the present invention to provide methods for decreasing aldehyde content of products in contact with polyolefin packaging materials.
It is another object of the present invention to provide methods for decreasing aldehyde content in polyolefin materials without coloring the polyolefin materials and without creating another source of product off-taste.
It is a further object of the present invention to provide polyolefin packaging containers which provide a decreased aldehyde content of products contained therein.
Methods are provided for decreasing the aldehyde content in polyolefin materials by combining the polyolefin material with an organic additive compound that reacts with aldehydes to form water and a resulting organic compound. The organic additive compound comprises at least two hydrogen-substituted heteroatoms bonded to carbons of the organic additive compound such that the organic additive compound is reactive with aldehydes present in the polyolefin to form water and the resulting organic compound, which comprises an unbridged five- or six-member ring including the at least two heteroatoms. The resulting organic compound neither imparts an off-taste to food and beverage products in contact with the polyolefin nor discolors the polyolefin.
The organic additive compound can be added at relatively low levels to the polyolefin and still sufficiently decrease the aldehyde content of the polyolefin. The organic additive compound preferably is substantially thermally stable at the melt processing temperature of the polyolefin. Preferred organic additive compounds include anthranilamide, 3,4-diaminobenzoic acid, malonamide, tryptophan, and allantoin.
It has been discovered that the off-taste from beverage and food products packaged in polyolefin materials is in fact largely due to the migration of C8 through C10 aldehydes from the polyolefin into the products, and that these aldehydes typically are the degradation products of various unsaturated additives used as processing aids in the polyolefins. These additives degrade chemically, thermally, or by photo-oxidation to yield these undesirable aldehydes. Examples of processing aid additives are erucic acid derivatives, such as erucamide, which often is used as a processing aid additive due to its efficacy and low cost. Although one might propose eliminating the processing aid additives as a solution to the aldehyde problem, the processing properties provided by the unsaturated additives are important and not readily duplicated. It thus is desirable to continue the use of unsaturated additives while simultaneously minimizing or eliminating the off-taste caused by their degradation. Methods therefore have been developed for substantially decreasing the aldehyde content of processed polyolefins by incorporating low levels of an organic additive compound into the polyolefin, such as during melt processing. The organic additive compound scavenges the aldehyde present in the polyolefin by chemically reacting with the aldehyde.
The Aldehyde-Containing Polyolefin
The present method is useful for removing aldehydes from a variety of polyolefin materials. In preferred embodiments, the polyolefin is polypropylene (PP) or polyethylene (PE). Other useful polymers include polyisoprene, polystyrene, poly(vinyl chloride), and poly(methylmethacrylate). The present method can be used with essentially any polyolefin polymer in which unsaturated additives have been incorporated.
Polyolefins, such as PP and PE, can be processed to form films, sheets, and three-dimensional articles, which can be used as or in packaging containers and closures. As part of the processing of the polyolefin, for example, melt-processing (e.g., extrusion and molding), processing aids typically are added to achieve favorable product properties and/or to improve ease of polymer processing. Representative types of processing aid additives include internal lubricants, external lubricants, and viscosity modifiers. Many of these processing aid additives are unsaturated compounds. Euramide, for example, often is added to polyethylene and polypropylene to act as a lubricant to improve handling and prevent the polymer product from sticking to itself. Other unsaturated additives that may generate the off-taste aldehyde include vegetable oil derivatives and castor oil derivatives. These unsaturated processing aid additives are known to degrade thermally, chemically, or by photo-oxidation to generate aldehydes in the polyolefin material. The aldehydes particularly identified as causing off-taste in products are octanal, nonanal, and decanal (i.e. the C8 through C10 aldehydes).
The Organic Additive Compound
Suitable organic additive compounds effective in the method described herein are characterized as small molecules that include at least two hydrogen-substituted heteroatoms bonded to carbons of the organic additive compound such that the organic additive compound is reactive with aldehyde in the polyolefin to form water and a resulting organic compound comprising an unbridged 5- or 6-member ring including the at least two heteroatoms. Unlike prior art methods that depend on the formation of inherently colored imines, the formation of unbridged 5- or 6-member ring structures do not inherently result in color formation. Moreover, thermodynamics often favor ring formation more than imine formation; therefore, significantly lower amounts of the organic additive compound described herein can effectively decrease the aldehyde content in processed polyolefins.
Heteroatoms capable of reacting with the aldehyde include oxygen (O), nitrogen (N), and sulfur (S). The heteroatoms of the additive compound should have at least one bond to an active hydrogen (H), and in the course of condensing with the aldehyde should split off water. Preferred functional groups containing these heteroatoms include amine (NH2 and NHR), hydroxyl (OH), carboxyl (CO2H), amide (CONH2 and CONHR), and thiol (SH). It is necessary for these functional groups to be sterically arranged so that upon condensation with the aldehyde an unbridged 5- or 6-member ring can be formed. The structural arrangement preferably allows the formation of a six-member ring. Furthermore, it is especially preferred that heteroatoms of the organic additive are attached to a preformed ring or rings. The preformed ring(s) most preferably are aromatic so that the unbridged 5- or 6-member ring of the resulting organic compound is bonded to the aromatic ring.
The organic additive compound preferably is substantially thermally stable at the temperatures required for melt-processing the polyolefin. The organic additive compound also preferably includes functional groups that include the heteroatoms and active hydrogens and that are relatively unreactive toward other additives, if any, present in the polyolefin. High thermal stability and low reactivity to other additives increase the amount of unreacted organic additive compound that is available for condensation with the aldehyde, thus reducing the amount of organic additive compound needed to achieve effective levels of aldehyde scavenging. Compounds with decomposition temperatures greater than 200xc2x0 C. as measured by Thermal Gravimetric Analysis (TGA) are most preferred. Compounds that decompose by intramolecular elimination reactions at temperatures less than about 200xc2x0 C. are less likely to be effective.
Examples of organic additive compounds that satisfy these parameters and that are effective at decreasing the aldehyde content of product in contact with polyolefins include anthranilamide, salicyclamide, salicylanilide, o-phenylenediamine, 3,4-diaminobenzoic acid, 1,8-diaminonaphthalene, o-mercaptobenzamide, N-acetylglycinamide, malonamide, 3-mercapto-1,2-propanediol, histidine, tryptophan, 4-amino-3-hydroxybenzoic acid, 4,5-dihydroxy-2,7-naphthalenedisulfonic acid disodium salt, biuret, 2,3-diaminopyridine, 1,2-diaminoanthraquinone, dianilinoethane, allantoin, 2-amino-2-methyl-1,3-propanediol, pentaerythritol, dipentaerythritol, and poly(vinyl alcohol). Preferred organic additive compounds include anthranilamide, 3,4-diaminobenzoic acid, malonamide, tryptophan, and allantoin. Anthranilamide and 3,4-diaminobenzoic acid are most preferred.
The amount of organic additive compound necessary to achieve the desired decrease in aldehyde content is dependent on which specific additive compound is used and the amount of reduction required. Organic additive compounds that are relatively more effective can achieve greater than 90% reduction in aldehyde content at loadings of between 200 and 500 ppm; additives that are relatively less effective may require addition levels up to 1000 ppm.
Using the Organic Additive Compound
The method of incorporating the organic additive compound into the polyolefin is not critical. For example, the organic additive compound can be dispersed in a liquid carrier which is then mixed with polyolefin pellets immediately before injection molding. The organic additive compound may also be incorporated by spraying a slurry of the additive in water onto the pellets prior to drying. In another embodiment, a melt or suspension of the organic additive compound is injected into pre-melted polyolefin. They also may be incorporated as a masterbatch pellet/pellet blend. The organic additive compound also may be incorporated by making a masterbatch of pellets of the additive compound with polyolefin and then mixing the masterbatch pellets with polyolefin pellets at the desired level before drying and injection.
The following equations illustrate the condensation reaction of suitable organic additive compounds described herein with an aldehyde (octanal) to form water and a resulting compound with an unbridged ring: 
In the foregoing equations, X-H and Y-H represent functional groups including at least one active hydrogen represented by H and a heteroatom such as O, N, or S. In equation 3, R1 represents a ring which can be a 5- or 6-member ring and can be aromatic or nonaromatic.
Using the Polyolefin Composition
Containing the Organic Additive Compound
The composition comprising the polyolefin and the organic additive compound is, because of the reduced aldehyde content, particularly suitable for making containers such as a container for use in packaging beverages. With the reduced aldehyde content, the containers impart less of an off-taste to the beverages. This is particularly important for beverages, such as water, which do not have a strong flavor. Particularly preferred polyolefins for use in beverage packaging include polyethylene and polypropylene. Containers can be made with the polyolefin compositions described herein using conventional methods such as injection molding and blow molding. A typical method includes forming a preform with the polyolefin and organic additive compound, and then blow-molding the beverage container. The resulting containers can be used in the manufacture of package beverages according to conventional manufacturing methods.